The present invention relates to enveloping a steel pipe or tube, possibly having a longitudinal welding seam. More particularly, the invention relates to improvements in the enveloping of such tubes or pipes using a thermoplastic ribbon which, for example, is being extruded right before being wound onto the respective tube or pipe.
The purpose of such an envelope generally is to provide for a protection of the steel pipe or tube if it is to be installed underground and in direct contact with the soil. The envelope may be made of an extruded polyethylene tape. Method and equipment for enveloping tubes and pipes have become known, in that, for example, a polyethylene ribbon or tape is extruded towards a pipe or tube, and a roll or pully guides the tape into immediate proximity of the tube while a pressure roll urges the tape into contact with the tube. The tube rotates on its axis and is also advanced axially so that a helical wrapping is obtained.
The surface of the tube may be provided with an adhesive, and the tube may rotate with a peripheral speed which is faster than the extrusion speed, so that the ribbon or tape as wound is stretched in the process.
The German Pat. No. 1,771,764 corresponding to U.S. Pat. No. 3,616,006 discloses, for example, a method wherein a thermoplastic ribbon and an adhesive ribbon are both wound helically on the rotating and heated steel pipe. Thermoplastic ribbon and adhesive ribbon are separately extruded, but just before the winding in either case, and the adhesive ribbon has properties of highly viscous tackiness at the working temperature.
Steel pipes or tubes with a longitudinal welding seam pose the specific problem that the plastic envelope as produced by means of one of the above described methods may not completely adhere to the pipe surface right at the welding seam, as that seam may have a slightly projecting bead, so that the otherwise cylindrical pipe's surface is uneven. As a consequence, capillary gaps are set up between the envelope and the tube or pipe, and corrosion producing substances may in some way find their way into these gaps.
In order to avoid this problem posed by the formation of gaps, the suggestion has been made (U.S. Pat. No. 3,814,646) to cover the welding seam prior to wrapping of the strip by means of plastic sections for obtaining a smooth interface between tape and pipe. Thermoplastic or thermosetting plastic in powdery, viscous or in form of longitudinal strips can be used here. The problem of the gap formation was solved indeed in that manner. It was found, however, recently that the enveloping of pipes by means of winding of tapes, ribbons or strips may exhibit another problem. It was found that the envelope is somewhat thinner right on the welding seam. On the other hand, the projection of the seam, possibly with additional cover, makes that particular portion of the tube particularly amenable to wear; the fact that the protective cover is thinnest here compounds the problem.